This invention generally relates to a sensor used for so-called electrical heating measurement in which a metal member is inserted in various liquid materials or semi-solid materials, the metal member is heated under the application of electrical current up to a higher temperature than that of the surrounding material, and changes in parameter of the metal member are continuously measured in relation to time lapse to detect changes in state of the material, and more particularly, it relates to this type of sensor which is workable for bending with high reliability and a manufacture method therefor.
The inventors of this application have already proposed, in Japanese Patent Application No. 60-197230, a sensor for electrical heating measurement comprising a core rod covered with an electrically insulating member, a thin metal wire wound about the core, and an electrically insulating member covering the thin metal wire.
Since in the proposed sensor the thin metal wire is wound about the core rod so that the length of the wire can be increased as compared to the usual straight wire to increase electrical resistance of the wire, the amount of generated heat per unit length of the sensor is increased proportionally, thereby making it possible to reduce the amount of current for obtaining required sensitivity. Further, for the sake of obtaining the same amount of heat generation, the length of the sensor can be decreased as compared to a sensor using the straight wire.
In addition, the thin metal wire wound about the core rod is improved in its shock proof properties and is not therefore liable to break. Also, the thin metal wire covered, interiorly and exteriorly thereof, with the electrically insulating members will not erode, for example, a tank for storing milk under the influence of electrolytic corrosion.
Accordingly, the curdy state of milk can be examined in a milk curdling process by inserting the sensor in the milk and by measuring temperatures of the thin metal wire in relation to time lapse while passing an electrical current intermittently or continuously through the thin metal wire.
The sensor constructed as above has many advantages but is still disadvantageous in that it does not have enough mechanical strength to be workable for stable bending. This is because during bending, pin holes are developed in the outer cover and its mechanical strength is degraded locally.